A strain wave gearing has a wave generator provided with a rigid plug and a wave bearing fitted on an elliptical outer circumferential surface of the plug. There are cases in which a roller bearing is used as the wave gearing. A roller bearing is constituted by an inner ring, an outer ring, a plurality of rollers accommodated between the rings, and a retainer having pockets for holding the respective rollers at constant intervals. Patent documents 1 and 2 propose strain wave gearings having a wave generator provided with a roller bearing.